Worm wheel



United States Patent WORM WHEEL Otto W. Ufert, Dusseldorf-Oberkassel, Germany, assignor to Schiess Aktiengesellschaft, Dusseldorf-Oberkassel, Germany Application March 21, 1955, Serial No. 495,563

Claims priority, application Germany March 30, 1954 4 Claims. (Cl. 74-458) The present invention relates to a worm wheel, especially a dividing worm wheel for gear cutting machines.

With high grade worm wheel drives, the formation of an oil film between the flanks of the worm and the lianks of a worm wheel contacted thereby is all the more 1mportant the higher the load will be which acts on the teeth or the higher the speed will be at which the worm and worm wheel slide upon each other.

In view of the fact that a good surface lubrication is of greatest importance for the working of a worm wheel drive, numerous suggestions have been made and tried out to facilitate the formation of an oil film between the flanks of the worm and of the worm wheel. Thus, for instance, attempts have been made to create favorable conditions for the formation of an oil film by profiling the worm n a manner which appeared to be appropriate, espectially by shaping the working surfaces in a certain manner. A change in the profile shape may indeed favorably affect the function of the drive.

The formation of an oil film, however, does not solely depend on the shape of the working fianks but also depends on the conditions under which the engagement between the worm and the worm wheel starts. With the known worm wheel drives, these conditions are generally very unfavorable because right at the start of the engagement between worm and worm wheel, the lubricating oil is stripped off at the edges of the teeth. In order to avoid this drawback, it has been suggested to remove the burrs of the tooth edges or to round ofi said edges. This step, however, is not suicient to obtain a noticeable improved oil film formation.

It is, therefore, an object of the present invention to provide a worm wheel which will overcome the above mentioned drawbacks.

It is another object of this invention to provide a worm wheel which is so designed that the stripping off of the oil film at the beginning of the engagement of worm and worm wheel will be avoided.

It is still another object of this invention to provide a worm wheel which is so designed that prior to the start of engagement between worm wheel and worm, a wedgeshaped oil film of sufficient length will form on the worm wheel which will prevent the stripping off of the oil and will promote the formation of an oil film between the worm wheel and the worm.

These and other objects and advantages of the invention will appear more clearly from the following specification in connection with the accompanying drawings, in which:

Fig. l is a diagrammatic side view of a gear cutting machine with a dividing worm wheel.

Fig. 2 is a cross-section through the Worm wheel according to'Fig. l'with teeth shaped in conformity with the present invention. the section shown in Fig. 2 being considerably enlarged over the illustration shown in Fig. l.

Fig. 3 is a partial section through the worm wheel according to the invention and illustrates the production of a vworm wheel according to the invention in conformity with the method differing from the method employed in connection with the manufacture of the worm wheel of Fig. 2.

Fig. 4 is a partial section through a worm wheel made according to still another method of the invention.

Fig. 5 is a perspective illustration of a worm wheel according to the invention.

Fig. 6 is a sectional view of the worm and worm gear in mesh along VI of Fig. 4.

Fig. 7 is a face view of the worm gear along VII of Fig. 4.

General arrangement According to the present invention, the lianks of the worm wheel are provided with working portions, while at both sides of said working portions there are provided portions which`will promote the formation of a wedge-shaped oil film. The flank portions are associated with worms, the axes of which are offset with regard to each other. The axes of the worms associated with said flank portions may be offset radially or laterally with regard to the worm wheel. The flanks of a worm wheel of this type may, in conformity with the invention, be cut in three steps. To this end, for instance, the working fiank portions may first be cut in customary manner, whereupon subsequently the tooth anks at one face of the wheel are cut by means of a tool which is offset toward said face, if desired, in radial direction from the work piece. Finally, the tooth iianks on the other face of the wheel are cut by means of a tool offset toward said last-mentioned face and, if desired, moved away from the work piece in radial direction.

It is also possible in conformity with the invention to produce the worm wheel flanks by two working steps. To this end, first the working ank portion is cut in a customary manner by means of a tool of the pitch diameter of the worm. Subsequently, the lateral portions of the tooth anks are cut by means of a tool which is removed from the work piece in radial direction and has a diameter greater than that of said first-mentioned tool.

The working operations are preferably carried out on one and the same machine, the tool axis only being adjusted accordingly. 1f the working operations are carried out in three steps, all three working steps may be carried out by means of one and the same tool.

Structural arrangement ,Referring now to the drawings in detail, Fig. 1 shows a gear cutting machine for cutting teeth into a heavy work piece Z which is carried by a face plate p, forming the work piece table. The face plate p is rotatably journalled on a bed. The rotation of the face plate p is brought about by a dividing worm wheel drive, including a dividing worm wheel r fastened to the face plate p, and also including a dividing worm s. In contrast to the heretofore customary shape and design of teeth for worm wheels according to which the entire tooth flank engages the worm, worm wheels designed in conformity with the invention have in addition to the normally shaped working tooth fiank portions a (Fig. 2) defined by the corner points C-D-F-E, laterally of said working tooth iiank portions a also free-cut or relieved flank side portions f', f" defined by the lines A--B-D--C and E-F-H-fG, These flank side portions f. f" may be produced in Vari.. ous manner.

According to Fig. 2. said portions f', f" are produced by hobbing means of the same hob by means of which the working flank portions a are cut. To this end, the

axis ofrthe cutter which, for producing the working fiank portions v:1,-occupiesthe working position W is Mvved into the positions W, and W2. ln these positions,

`the-axis of the tool is over thepositionoccupieddurin Patented Feb. 4, 1958l the. cutting of the flank center portions-a oisetby` small distances X', X and Y toward the two faces of the wheel or in radial direction away from the wheel center point. The cut-free surfaces f', f" themselves-'thus form portions of teeth fianksofa worm-wheel'. When -freecutting the flank portions f', f by means of a cutter having a diameter R and by offsetting the axes" by" X orX and Y, alsothe tooth base is free-cut inconfo'rmitywith the-shape' of a commaas -is indicated by the -points B-B-D and H-H'-F. Byaxiallyfadjusting thehob in sucha manner thatV the' cutterteeth` cansimultaneously work on the front and backside of` the tooth' anks, thetadvantage is obtained that' by means of' a'single'cut, the total work of the free cuttingof one side of the worm wheel at the front and reartooth-'anks and at the base of. the tooth can be carriedout;

As shown-in Fig. 3, thefree'surfaces ff,;f" can also` be. created if, followingfthe cuttingof the; cuttingqankl portions a by means of a cutter of ,the radiusR, the axis, of which is designated with the referenceW, a llarger tool of the radius R' is moved away1 from the workpiece byra distance Y and has its axis moved-,into therposition. W3. This method is particularly expedient wheni` cut.- tingfby means of an impact tooth. Itis self-understood that also in this instance, the greatertoolzhasto be' so adjusted that itwilly attackuniformly at Aallzpointsitofbe cut free.

instead of. a greater work tool oraiworhtoohof; the

same size,` also a smaller work toolmay beaemployed-for producing the free cut surfaces f",`f".. This isrclearlyfili lustrated in Fig. 4 according to.which"followinglthe.' crc.L ation of the working surfacesa by means roff aztool vofa radius R, the axisof the tool is onlyrlaterallyhmoved to ward the faces of the work piece by a distance lX.` In other words, the axis of theY tool is movedfout ofthel position W into the positions W4 or W5. Inthesepositions,:

another worktoolis employed which fhas-a-radiuslRW smaller4 than the radius R of'the rst employed tool.

The'ernployment of. a worm wheelimade according-to theinvention in. a' worm wh'eeldrive thusbringsaboutA the formationofa longitudinally extending wedge-shapedv oil lm with a small wedge -angle outside the 'range 'ofi engagement of said worm wheel withthe respective worm.

Thus,'at the veryI startI ofthelengagement between/worm and; worrrrwheel,1 wedge-shaped lubricant cushions are presentiwhichimake'impossiblefthetearingk oi of the oil`A film and. will `assure that also-within `the range'A'of-enL gagement between worm and worm wheel, said oil lm will not vanish. In vaddition thereto, the advantage is obtained thatthefteethzdueo the increase-ofi-.their lateral grip'are more bent-resistant than theteeth ofag worm wheel'having uniform workingtsurfaces;I This-Lisoip'arf ti'cularrimportance in connectionwith worm wheelsfwhich" in view, of specialworkin'g'con'ditions'ofthe drivel ernbrace, the worm over a wide angleas is the-case,`forin stance,.with dividing. worm wheels ofl large gear cutting machines which have a relativelyfneltoothing andi are provided with a considerable'.numberfoffteetl'l,I so'that at the; last position before the workf piece," once-mo're `a great` transmission ratio'.can-be.obtained: YThese dividinglworm wheels have to `move Withgreatestruniformity abputfsthattf-thterangeaof; eugagementiwith the worm ex# tends `over a great sector angle of the dividing worms.. and thus has a form which is unfavorable for the formation of an oil lm. The engagement also becomes very long and, consequently, the difference in the heat extension co-eiiicients of worm wheel and worm becomes all the more eifective in a harmful manner the longer the range of engagement. In thisy connection, it should be noted that with gean cutting machines, the worm and worm wheel respectively consist of different material, and that during the operation of the drive, a considerable increase in temperature of said parts will occur.

The invention, on they other hand, makes possible the production of rigid worm wheel teeth adapted to resist high load'without enlarging the range of engagement between the worm 'wheel and the worm. The working lianks can be limited to the central portion of the Worm wheel profile.

The formation of a high" oil pressure and thus the requiredoil'lm at the start of the engagement between worm wheel and wormY is furthermore favored by the fact that that portion of the' tooth bottom which delimitsV the ank side portionsisntapered in a continuously increasing manner in the direction toward the. faces of the wheel.

With theworm wheel' according to the invention, the oil wedgeangle 4remains substantially uniform over the entire height'of the tooth.

Furthermore, the creation of the sections to be free cut is considerably simplified because the free cutting of the flank side portions `and the tooth bottom can be effected on the same machine with the same tooland while the'work piece remains chucked in its respective position in which the cutting ofthe working flank portions of the teethY was effected. Special'machine tools, specialr tools and additional! operations .caused by a resetting of the work piecev and'rechucking of the. same are not required.

It is, of course,`understood 'that' the present invention is by no means limited to the particular construction shown in the drawings but also comprises any modifications within the appended claims.

Thus, also worm wheels which'instead of/a cylindrical blank shape are conically shaped can be provided with free cut surfaces in conformity with the present invention. Furthermore, instead of a hob for the free cutting, also an impact knifemay be employed. The free cutting may be etfected asymmetrically Vwhen theworm wheel is not symmetrically designed.

What I "claim'is:V

l. An oil lrn retaining woirn wheel having its tooth anks provided with workingsections and'being provided at bothends of said working sections with portions cut at .such a radius that the circle describedI thereby forms at-leach of said ends anl acute spherical angle with the circle describedby the radius at which the-respective adjacent working, section has Vbeen cut, the apex of each of said angles being located substantially at the innermost point-` of the end edges of the respective adjacent working section,v thereby creatingl sections which when engaged byl a wormv will retairra wedge-shaped oil lm at each end of each'of said working sections.

2. An oil film retaining worm wheel having its tooth flanks/providedl with-working `sections and being provided at 'both ends'- of said working sections with oil retaining sections adaptedv when meshing with a worml to retain a wedge-shaped oil lm, said oil retaining sections having curvedy surfaces with the centerv of curvature thereof offset with regard to the center of curvature of the pitch circle of said worm wheel.

`3. Anoil iilmretaining'worm wheel having its smooth flanks. provided with-working sections andbeing. providediat both endsof Asaid working sections with oil retainingsections adapted when meshing with a worm to retaina wedge-shaped'oil'lm, said oil retaining sections having curved surfaces withVV the center of curvature" 5 thereof oiset with regard to the center of curvature of the pitch circle of said worm wheel in radial direction of said worm wheel.

4. An oil film retaining vworm wheel having its tooth flanks provided with working sections and being provided at both ends of said working -sections with oil retaining sections adapted Awhen meshing with a worm to retain a wedge-shaped oil ilm, 'said oil retaining sections having curved surfaces with the center of curvature thereof offset with regard to the center of curvature of the pitch circle of said worm wheel in lateral direction of said worm wheel.

References Cited in the file of this patent UNITED STATES PATENTS Merritt Feb. 27, 1934 Scott Mar. 26, 1940 Steiner Feb. 16, 1954 Staples Jan. 25, 1955 FOREIGN PATENTS Great Britain Mar. 30, 1955 

